I ran into this thread
on another forum I visit where the topic is acceptable tolerances in an aluminum dog manufactured to fit into a 20mm hole arranged on 96mm centers(unlike Harry, I'm bilingual when it comes to measurements
I really liked the manufacturer's comments in a reply: "The talk of tolerances comes up often on this forum and I think it needs some clarifying. Hopefully no one here is trying to make a 3 foot square panel and hold it to .005" tolerance, it just can't be done on a repeatable basis. I would think anything beyond 1/64" (.016") is pushing your luck (this is mostly due to the machines we use, not the wood). But if you're making a dado for a panel to fit in, then 1/64" is very loose and most would find unacceptable for gluing. If you were to make a 1/4" bolt hole in the panel, you may not be happy with 1/64" tolerance."
"When it comes to tolerance for being square, saying it's out by 1/8" doesn't tell anybody anything. We need a length to go with that 1/8". There's a big difference between being out of square by 1/8 over a 12" length and by 1/8" over an 8 foot length. You should always include a length."
Since World War I (nearly 100 years ago) machinists have understood the need for standards and tolerances. Because of this parts can made in separate parts of the country and still be assembled together with exacting fits. Yet woodworkers continue to buy rulers that are not marked correctly and squares that are not square (would you buy a saw that could not saw???) and it is all done with the excuse that tolerances don't matter with wood because wood moves. That "excuse" should be the exact reason to demand accuracy, not ignore it. Think of all the work that is redone by woodworkers because one guy's ruler was off from the other guys, or something was built to fit a square corner but at install time they discover the corner wasn't square. All of this can easily be stopped by demanding better tolerances from your basic tools.
[end of rant] "
I see this all the time on all the woodworking forums I visit, tight tolerances aren't important because "wood moves". No argument with the "wood moves" part, it does and will continue to do so. But it moves consistently in known ways and that movement can be predicted and accounted for in the design stage of any project.
Using that as a justification for buying poor measurement tools to save a few bucks and not spending the time to learn good measurement practices is just a justification for sloppy work, IMHO.
And what about jigs and fixtures? Good MDF is held to very tight tolerances and doesn't move nearly as much as solid-sawn lumber, good plywood is a close second. I have fixtures and jigs made of MDF, Lexan and precision-ground cast aluminum, they're all accurate to at least .001" in 18" and made with ordinary woodworking tools. So don't tell me it can't be done. The wood that comes out of them is equally accurate, as long as everything is held to close tolerances the finished assembly can be expected to come out as designed with no surprises or refitting.
Real-world example, a typical cabinet door 12" wide, 30" tall in a 24" wide cabinet: Let's say my crosscut on the rails is 1/128"(.008") out of square in 2-1/2 inches of width. Now that 1/128" is negligible because wood moves, right? I clamp that rail tight against the 30" stile, that .008" deviation from square is now 12 times that deviation at the end of the stile(30/2.5=12) so now one side of my door is .096"(3/32") out of square. But wait, there's more!
The other side of the door is out an equal amount so I now have a door that's racked 3/16". Now I have the task of trimming the doors on each side from nothing to 3/32" just to square them up. So I finally figure out how to do that but now my perfectly sized doors are 3/8"(3/16"x2) short of the cabinet width they're supposed to fit into.
But that's OK. I'll just tell my customer "wood moves". Right.....